Presently employed electrochemical gas sensors are either of the potentiometric or the polarographic type (including other current measuring approaches).
Potentiometric sensors are basically modified pH-electrodes whose operating principle is based on the measurement of a potential that is effectively a linear function of the logarithm of H.sup.+ ion concentration. Typically, electrochemical sensors for carbon dioxide, hydrogen sulfide, nitrogen oxides, sulfer dioxide, etc., belong to this category. As an example, the relationship between pH and the partial pressure of carbon dioxide (P.sub.CO.sbsb.2) in the sensor is EQU .DELTA.pH=0.97 .DELTA.log.sub.10 P.sub.CO.sbsb.2.
Since one unit of pH at room temperature corresponds to less than 60 mV potential change, it is obvious that this type of measurement is intrinsically inaccurate (a change of P.sub.CO.sbsb.2 by a factor of 2 would result in a 18 mV change). Furthermore, whether a glass electrode or other types of H.sup.+ electrodes such as a palladium/palladium oxide, iridium/iridium oxide, or a solid state ChemFET H.sup.+ electrode is used, this type of sensor has the inherent difficulties of pH-electrodes: low-sensitivity, slow response time, and considerable drift.
Polarographic type sensors and other sensors based on a current measurement have a two-electrode or a three-electrode configuration. In either case, the working principle of this type of sensor is based on the measurement of a current, usually the limiting current, of an oxidation (or reduction) reaction involving the gas to be measured. The magnitude of this current is made proportional to the partial pressure of the gas in the environment. Typically, electrochemical sensors for oxygen, carbon monoxide, and alcohol belong to this type. The major problems of this type of sensor are low sensitivity and poor selectivity.